Metallocene catalysts have been used to manufacture polyolefins for many years. Countless academic and patent publications describe the use of these catalysts in olefin polymerisation. Metallocenes are now used industrially and polyethylenes and polypropylenes in particular are often produced using cyclopentadienyl based catalyst systems with different substitution patterns.
The most important physical properties of isotactic polypropylene (iPP) are its average molecular weight and its melting point (Tm), the latter being mostly determined by the degree of stereoregularity (isotacticity) and regioregularity/total chain defects of the polypropylene chains.
The Ziegler-Natta catalyst systems known in the literature can produce iPP with high molecular weights together with moderate to high isotacticities and melting points (Tm). The Tm (measured by standard DSC methods) of non-nucleated iPPs are in the range of 160 to 165° C.
Since their discovery and for the following ten years of development, metallocene catalysts for polypropylene have been limited in their application because of low activity, their limited molecular weight capability and the relative low melting and low stiffness of the PP homopolymer they could produce. Since 1992, due to improved ligand design, several families of bridged bisindenyl metallocene catalysts have been described, that were able to produce polypropylene homopolymers with increasingly higher molecular weight and higher isotacticity.
However, in the case of metallocenes, there are very few examples which can produce iPP having both very high molecular weights and high melting points. For example rac-Et(2,4,7-Me3Ind)2ZrCl2 can produce isotactic polypropylene with a molecular weight of 1,900,000 g/mol and a melting point of 168° C.
The most successful ligand types are based on the basic 2-methyl-4-aryl-indenyl substitution pattern: for example, rac-Me2Si(2-methyl-4-phenylindenyl)2ZrCl2 was shown to produce homo-PP with a relatively high melting point of 150-151° C. and fairly high molecular weight even at industrial polymerization temperatures. However, these complexes quickly lost their molecular weight capability as soon as ethylene was added to the system, so were unable to produce C2-rich random copolymers or heterophasic copolymers of the appropriate molecular weights.
One solution found to increase the molecular weight of copolymers has been to replace one of the two 2-methyl groups with a branched alkyl group, such as 2-isopropyl. This substitution pattern, which generates a C1-symmetric complex, led to a slight increase in isotacticity of the homopolymer and a marked increase in the molecular weight of the ethylene-propylene copolymers compared to the C2-symmetric rac-Me2Si(2-Me-4-PhInd)2 ligand system.
However, in all cases the increase in molecular weight of the copolymers was obtained at the expense of activity, or catalyst cost, or both.
The present inventors have found that by using a suitable combination of indenyl ligands where both indenes are 2-methyl substituted, and preferably by using particular single site catalyst technology, that ideal polymer properties can be achieved.
The invention also covers a new and improved synthesis of key intermediates needed in the synthesis of the catalysts of the invention.
The catalysts of the invention are new although similar catalysts are of course known in the art. The metallocene rac-Et(2,4,7-Me3Ind)2ZrCl2/MAO is known. In U.S. Pat. No. 7,405,261, rac-Et[2,7-Me2-4-(4-tBuPh)Ind]2ZrCl2 is reported to produce iPP with a melting point of 156° C., by polymerizing liquid propylene at 65° C.
WO2009/054831 describes zirconocenes with a 2-methyl-4,7-aryl substitution pattern, such as rac-Me2Si[2-Me-4,7-(4-tBuPh)2Ind]2ZrCl2. The melting points of the homopolymers are still quite low, being in all cases below 150° C. despite the relatively low polymerization temperature of 65° C.
WO02/02576 describes conventionally supported metallocenes such as rac-Me2Si[2-Me-4-(3,5-tBu2Ph)Ind]2ZrCl2. These metallocene catalysts, activated with MAO or a borate, on a silica support, at a polymerisation temperature of 60 or 70° C., give iPP with Tm between 156 and 159° C.
The metallocene rac-9-silafluorenyl-9,9-[2-Me-4-(3,5-tBu2Ph)Ind]2ZrCl2 also gives high melting point iPP and is described in WO02/02575.
All the above examples are based on C2-symmetric metallocenes, that is those in which both indenyl ligands are identically substituted. The present invention however, is concerned with asymmetrical ligand structures.
There are also several examples of isoselective bisindenyl metallocenes having C1-symmetry, that is metallocene complexes in which the two bridged indenyl ligands have different substitution pattern.
Spaleck et al. in Journal of Molecular Catalysis A: Chemical 128, 1998, 279-287 describes some bisindenyl catalysts which are asymmetric but which lack any substituents on the 6 or 7 position of the 6-membered ring. These complexes, although of relative simple structure, have a quite poor performance in propylene polymerization.
In WO2005/105863 and WO2004/106531, various asymmetric catalysts are disclosed which have a branched alkyl group at the 2-position of the ring. Such catalysts have poor activity. WO2001/048034 also requires branched structures at the 2-position of the metallocenes therein.
EP-A-1692144 describes asymmetrical catalysts based on tricyclic rings.
The present inventors seek alternative asymmetrical catalysts that can allow the formation of interesting iPP and copolymers of polypropylene at high catalyst activities. Also, in all the above cases, the preparation of the indenes require multistep syntheses which render the ligands quite expensive.
The catalysts of the invention comprise an optionally substituted aryl or heteroaryl group at the 4-position of the indenyl ligands and a linear hydrocarbyl substituent at the 2-position of the indenyl ligands. On one ligand of the catalyst there is a 6 or 7-position group, however, the other ligand does not carry a 7-position group. The metallocenes of the invention are asymmetrical so it is essential that the two ligands differ. Benefits achieved by using catalysts above include random copolymers with higher molecular weight and, in particular, polymerisations in which the catalysts exhibit higher activity. Also, it is believed that the catalysts of the invention allow a higher degree of fine-tuning of their polymerization performance, compared to the conventional symmetrical catalysts.
It is a further preferred advantage that the catalysts of the invention are easy to synthesise.